'''Thermal mass''' is a heat storage material, such as water, concrete or
masonry, used in [[solar thermal energy]] systems also known as [[passive solar heating]] systems. With passive solar heating, the house itself acts as the solar collector and the storage facility. The essential elements of a passive solar system are:
* excellent [[insulation]],
* [[solar collection]] (with south-facing windows), and
==Types of passive systems==
There are three categories that passive systems can fall into
Finally, the third category of these passive solar heating systems is an attached greenhouse on the south side of the house. The greenhouse acts as an expanded thermal storage wall. It will share some of its heat with the adjoining house.